1. Field of the Invention
The present invention relates to an optical pickup device, and more specifically, to a single surface holographic optical element (HOE) that implements the functions of generating satellite beams to be used for tracking and creating a focus error signal.
2. Description of Prior Art
Referring to FIG. 1 (prior art), a conventional optical pickup device, which is similar to that disclosed in Japanese Application Laid-Open No. 56-57013, is illustrated. In the figure, a recording medium 1, such as an optical disk, consisting of a number of recording tracks 10 is provided for data memory element. A light source 11 which is generally a semiconductor laser is also provided for generating a light beam 16. Between recording medium 1 and light source 11, a condensing lens 14, a holographic beamsplitter 13, and a diffraction grating 12 are aligned in parallel. Light beam 16 emitted from light source 11 is diffracted by diffraction grating 12 into three light beams. Each of the diffracted light beams passes through holographic beamsplitter 13 and condensing lens 14 and then shines on recording tracks 10 of recording medium 1. Three light beams reflected from recording tracks 10 of recording medium 1, reversely, pass through condensing lens 14 and are diffracted by holographic beam splitter 13 which causes them to change their direction to a photodetector 15 rather than light source 11.
Since the holographic beamsplitter and the diffraction grating are provided individually, the number of device elements is increased and more alignment tasks are required for the aforementioned optical pickup device. In this respect, it is desirable to have optical pickup devices having a single diffracting element which implements the functions of both the holographic beamsplitter/focus error generating device and the diffraction grating for generating the satellite beams used in track following. For example, the device structure illustrated in FIG. 2 (prior art), which has been disclosed in U.S. Pat. No. 5,111,449, consists of a recording medium 1, a light source 21, a diffracting means 22, lens 23 and 24, and a detector 25. There are a number of parallel tracks 10 on recording medium 1. A partition line defines diffracting means 22 into a first region 22a and a second region 22b. First region 22a has a diffraction grating for diffracting a light beam 26 from light source 21 into three beams A1, A2, and A3 toward recording medium 1 through lens 23 and 24. Beams A1, A2, and A3, which are a +1 order, a -1 order and a zero-order light beam respectively, shine on points P1, P2, and P3 of recording medium 1. Then their reflected light beams B1, B2, and B3 from recording medium 1 are diffracted by second region 22b of diffracting means 22 toward detector 25.
However, main light beam A3 of the optical pickup device is a combination of two zero-order light beams which are diffracted by first region 22a and second region 22b of diffracting means 22. The two regions will affect the intensity of the two halves of zero-order diffracted beam A3 in different ways thereby disturbing its ideal beam profile. Since the zero-order diffracted beam is used for the most critical function in the pickup head, namely reading the information stored on medium 1, its beam quality should not be degraded. Thus, a shortcoming of the approach depicted in FIG. 2 (prior art) is that it will affect the beam quality of the zero-order beam A3 and therefore degrade the quality of the information signal read by the pickup. Moreover, since only one half each of the reflected beams B1-B3 is diffracted toward detector 25, the round-trip efficiency from laser to detector is poor. As such, the optical pickup device has to be provided with a higher power light source to overcome this problem. Therefore, the pickup device consumes more power, requires more device space and is more expensive to manufacture.